The present invention is related to the invention disclosed in applicant's prior U.S. Pat. application Ser. No. 07/362,973, filed Jun. 8, 1989, now abandoned. It is directed to an improvement in the monitoring of atmospheric environments. More particularly, the invention relates to an improved corrosion test element and method for using the test element to determine the rate at which atmospheric corrosion takes place in a designated environment.
The present invention was developed in response to the need for a method for quickly and inexpensively measuring the rate of corrosivity of an industrial environment without requiring complex and time consuming analysis and/or manpower to obtain such results. The standard industry method currently available employs the exposure of specially prepared copper coupons. After exposure to a selected environment for a specified time period, the copper coupons are cathodically (electrochemically) reduced in an electrolyte, requiring complex glassware and instrumentation, and the time of a highly skilled technician. This method is time consuming, labor intensive and hence expensive. At least two alternate methods exist for measuring atmospheric corrosivity, one employing the measurement of the resistance of a thin pathway of copper, and the other employing vibrational frequency measurement of a copper coated quartz crystal (quartz crystal microbalance). However both of these alternatives require expensive sensors, sophisticated instrumentation, and databases relating the measured parameter change to the actual corrosion or metal disappearance rate. Obviously it would be far preferable to have available a less expensive corrosion indicator that would provide evidence of a corrosive atmosphere on a more timely basis, and which would allow the user to calculate the rate at which the corrosion is taking place.
Corrosion test elements have been used for testing the corrosion rate of metals in the presence of corrosive liquids. In U.S. Pat. No. 2,351,644, there is disclosed a test element having a quantity of the metal which is to be tested applied to a suitable carrier. The test element is placed in contact with a corrosive liquid for a designated period of time, and upon removal, the amount of the test metal removed is noted. From this test, the rate of corrosion of the metal can be calculated. In a related invention disclosed in U.S. Pat. No. 3,084,658, a moisture and corrosion indicator includes a transparent disk onto which there is applied a sensing material either as a film of cobaltous chloride or a metallic material. When exposed to moisture, the cobaltous chloride's color changes from blue to pink and then back to blue when the moisture is removed. Meanwhile, when a metallic material such as pure iron is exposed to moisture, it will rust, and the rusted surface may be seen visually when looking through the transparent disk. However, neither invention contemplates a method for determining the rate of corrosivity of an atmosphere. In the present invention, the rate of corrosivity for an environment can be determined by visually observing the rate of change in the area of the reflective surface of a corrodable metal film applied to a transparent substrate over time, as viewed through the substrate, or, when a security coating is applied to the substrate, by observing the change in reflectivity through the substrate using a suitable instrument which emits radiation in a frequency range which is not reflected by the security coating.